Insurers enjoy second-biggest six months of municipals business.

The municipal bond insurance industry guaranteed $23.13 billion of new issues in the first half of 1991, the second-larges six-month volume of municipal insurance in the history of the tax-exempt market.

Over all, teh bond insurers backed more than 31.2% of long-term new issuance, marking the greatest influence the industry -- comprising only five companies -- has wielded over the municipal bond market in the last six years. In the second half of 1985, the six-month zenith, $29 billion was guaranteed, according to Securities Data Co.